What to check on a Magnetic Compass (TMC).
A gyrocompass finds true north electrically, by sensing the earth's rotation, and needs power and settling time after a blackout before it can be trusted again. A magnetic compass needs neither: its card, floating in a liquid-filled bowl, aligns itself with the local magnetic field the moment it is free to move. That reliability under total power loss is why the magnetic compass remains a mandatory, physically separate piece of equipment rather than a redundant gyro repeater. Its reading is magnetic heading, not true heading, and it must be corrected…
What sets this instrument apart
A gyrocompass finds true north electrically, by sensing the earth's rotation, and needs power and settling time after a blackout before it can be trusted again. A magnetic compass needs neither: its card, floating in a liquid-filled bowl, aligns itself with the local magnetic field the moment it is free to move. That reliability under total power loss is why the magnetic compass remains a mandatory, physically separate piece of equipment rather than a redundant gyro repeater. Its reading is magnetic heading, not true heading, and it must be corrected for the ship's own magnetism before it means anything to the navigator.
Main components
Compass bowl and card
The card, carrying the compass rose and pivoted on a jewelled bearing, floats in a mixture of distilled water and alcohol chosen to stay fluid across the ship's operating temperature range and to damp out the card's swing in a seaway.
Binnacle and correctors
The bowl sits gimballed inside a binnacle that also carries the compensating magnets, the Flinders bar for correcting heeling error, and quadrantal spheres that counter the ship's induced magnetism. These are set during a compass swing and left undisturbed until the next one.
Transmitting unit
A separate sensor, usually a flux-gate or an optical pickup reading the card's position, converts the magnetic heading into an electrical signal for repeaters, the autopilot and the integrated bridge system — this is the "transmitting" part of a TMC, distinct from the analogue standard compass that is read by eye alone.
Regulations and class
SOLAS Chapter V regulation 19 requires a magnetic compass, or a means of determining heading by magnetic compass, on ships regardless of the navigation equipment otherwise fitted, along with a means of correcting heading for error. The compass must be adjusted by a qualified compass adjuster after major structural or magnetic changes near its position, and the resulting deviation card is kept on the bridge and checked against the log at survey. Class and flag state both expect the deviation card to be current, not a document that has quietly outlived a refit.
Typical faults
| Fault | Consequence |
|---|---|
| Deviation table left unrenewed after steelwork or new equipment near the compass | Bridge steers to an error the card no longer describes correctly |
| Bubble forming in the compass bowl from a slow liquid leak or temperature cycling | Card sticks or swings erratically, especially in a seaway |
| Gimbal wear or corrosion | Card no longer stays level as the ship rolls, degrading reading accuracy |
| Transmitting unit failure or misalignment with the card | Autopilot and repeaters receive a heading that disagrees with the compass actually read on the binnacle |
What to look for in a supplier
- Compensation range of the binnacle's correctors relative to the ship's expected magnetic disturbance
- Transmitting unit output format matched to the bridge system already installed (synchro versus digital interfaces)
- Ease of access for a compass adjuster to swing and correct the installation without dismantling the binnacle
- Liquid and bearing serviceability without full compass replacement
Practice
Officers should compare the standard compass reading to the gyro heading each watch as a matter of routine, not only when the gyro alarms, since a slow transmitting-unit drift produces no alarm at all.
Anything ferrous placed near the binnacle — a tool box, a phone, a replacement fire extinguisher — can shift the deviation enough to matter; the compass adjuster's card assumes the binnacle's surroundings stay as they were on swing day.